Heat-resistant resins such as polyimide resin are excellent in heat resistance and mechanical properties; thus, the resins have already been used widely, as a surface protective film and an interlayer dielectric of a semiconductor element, in the field of electronics. Recently, attention has been paid to screen printing or dispense coating, which does not require any complicated steps such as light exposure, development or etching, as a method for forming an image of a polyimide resin film for the surface protective film, the interlayer dielectric, a stress relaxing material, or the like.
In screen printing or dispense coating, a heat-resistant resin paste is used which contains, as constituting components, a base resin, a filler and a solvent and has thixotropy. In almost all of heat-resistant resin pastes that have been developed so far, an inorganic filler such as silica filler, an insoluble polyimide filler, or some other filler has been used as a filler for giving thixotropy. However, when the paste is heated and dried, the filler remains, as a filler, in the coating. For this reason, the following problem is pointed out: a large number of voids or air bubbles are easily generated in the interface between the base resin and the surface of the filler, so as to lower the coating strength; thus, the electric insulation is poor.
Against the problem, a heat-resistant resin paste is developed wherein a base resin and a solvent are combined with a soluble organic filler which forms, together with the base resin, a homogeneous phase without remaining as a filler in the coating when the paste is heated and dried (see Japanese Patent No. 2697215, and Japanese Patent No. 3087290). However, the formed coating is transparent in many cases since a soluble organic filler is used in the heat-resistant resin paste. Thus, there remains a problem that the coating is poor in viewability when the coating is patterned, so as to give a low workability.
An object of the invention is to provide a heat-resistant resin paste excellent in heat resistance, flexibility, printability, and viewability.